bug#45029: 27.1.50; Regression: Yanking into externally modified file with delete-selection-mode

[Juri Linkov <juri@linkov.net>]

>> Here is a brief trace that explains the cause of the problem:
>>
>> 1. C-y (yank) calls delete-selection-pre-hook from pre-command-hook
>> 2. delete-selection-helper calls delete-active-region
>> 3. this activates ask-user-about-supersession-threat on a modified file
>> 4. it calls read-char-from-minibuffer ('this-command' is still 'yank')
>> 5. read-char-from-minibuffer waits when user types 'y'
>>    (bound to the command 'read-char-from-minibuffer-insert-char')
>> 6. after typing 'y', read-char-from-minibuffer returns 'y', but
>>    now 'this-command' is 'read-char-from-minibuffer-insert-char'
>> 7. when delete-selection-pre-hook finishes, due to 'this-command'
>>    'read-char-from-minibuffer-insert-char' is called again,
>>    and this time operates on the buffer as if it's the minibuffer
>
> Yuck!
> And thanks for the investigation.
>
>> Anyway, here is a patch with two fixes:
>> 1. Guards read-char-from-minibuffer-insert-char against inadvertent
>>    operating on the non-minibuffer buffer;
>
> I suspect this will/would just hide a problem (such as the current one)
> under the rug.

I admit this is a workaround, but I have no idea what would be a proper fix.

>> 2. Prevents read-char-from-minibuffer from changing the value of
>>    'this-command' by read-from-minibuffer:
>
> Right, but I think that this needs to apply to all recursive edits
> rather than only `read-char-from-minibuffer`.

I tried to test recursive commands at the read-char-from-minibuffer's
prompt, but can't reproduce the same problem.